The present invention relates generally to methods and an engine for reduced diesel emissions.
The stringent emissions regulations imposed by United States and European regulatory agencies have progressively reduced the amount of emissions allowed in the exhaust gases of diesel engines. Recent proposals from regulating bodies have mandated that engine manufacturers maintain a maximum NOx emission of 30 g/hr during extended idling conditions.
Current known emissions control strategies are unable to achieve this target without damage to engine components. For example, current methods of implementing exhaust gas recirculation (EGR) at idle will tend to cause EGR cooler fouling and engine sludge and varnishing. Repercussions from this can vary from component replacement to complete engine failure. Also, during engine idle, there is a danger of after-treatment system damage. The cooler exhaust temperatures and higher hydrocarbon environment can cause Diesel Oxidation Catalyst (DOC) plugging, and the DOC and Diesel Particulate Filter (DPF) can both collect a combination of particulate matter, hydrocarbons, and water during extended periods of idling. This collection can cause extreme back pressure leading to engine damage or after-treatment damage and, if there is enough hydrocarbon present in the system, when the exhaust gas temperature rises to a sufficiently high level, the unburned fuel can ignite and cause the after-treatment system to melt down or crack, thereby destroying it.
Also, the combustion temperature in a diesel engine is strongly influenced by the intake air temperature and the air/fuel ratio. At a low thermal state, the hydrocarbon emissions resulting from incomplete combustion and lubricating oil passage or “slobber” will have an increased tendency to condense because the surfaces are cool and the hydrocarbons are of higher molecular weight than usual for a hotter combustion. The low thermal state is usually associated with high A/F ratios, low ambient temperatures, and low fueling rates leading to both low combustion temperatures due to the chemical kinetics and a low “bone” temperature (core metal temperatures) absorbing a larger proportion of the heat energy and resulting in an additional lowering of combustion temperatures. Although this condition is not mechanically stressful to the engine, it does result in less complete combustion and more hydrocarbon emissions. The low thermal state of the engine promotes the condensation of water, which is a byproduct of all combustion. The water dissolves sulfur and nitrogen compounds thus creating an acidic solution. The coating of liquid hydrocarbons and acids onto a layer of carbonaceous soot can render the EGR cooler ineffective as a result of diminished heat transfer efficiency (fouling) and may also incur plugging which restricts EGR gas flow area.
While operators might connect their vehicles to external or auxiliary power supplies in some circumstances, instead of idling their engines, to do so it is necessary that the vehicle be near an external or auxiliary power supply. These methods inherently reduce emissions but the primary purpose for their implementation is to reduce fuel consumption of the vehicle.
It is desirable to provide a technique for reducing engine emissions when the engine operates at low loads or idles. As there is no “throttle” in most diesel applications, idle can be considered to be the condition when the engine operates at a load required to overcome friction torque and accessory loading from engine and chassis systems.
According to an aspect of the present invention, a method for controlling emissions during low-load diesel engine operation is provided, the engine comprising at least one piston movable in a cylinder between a top dead center and a bottom dead center position, a fuel injector for injecting fuel into the cylinder, and a variable geometry turbine through which exhaust from the engine is adapted to flow. According to the method, the engine is operated at low load, and a variable geometry turbine inlet opening size is controlled so that NOx emission levels are controlled while the engine is operated at low load.
According to another aspect of the present invention, a diesel engine comprises at least one piston movable in a cylinder between a top dead center and a bottom dead center position, a fuel injector for injecting fuel into the cylinder, a variable geometry turbine through which engine exhaust is adapted to flow, a sensor for measuring NOx emissions at an exhaust of the engine, and a controller arranged to control a variable geometry turbine inlet opening size during low load operation of the engine responsive to NOx emissions measurements so that NOx emission levels are controlled.